1. Field of the Invention
This invention relates to methods and apparatuses for liquefying natural gas. In another aspect, the invention concerns a liquefied natural gas (LNG) facility employing a system for enhanced fuel gas composition control.
2. Description of the Related Art
Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form.
Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers.
Storing natural gas in its liquefied form can help balance out periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream via indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, acid gases, nitrogen, and ethane and heavier components) from the natural gas stream at different points during the liquefaction process.
The cooling required by LNG facilities to liquefy the natural gas stream is typically provided by one or more mechanical refrigeration cycles. These mechanical refrigeration cycles generally employ one or more refrigerant compressors, which are usually driven by gas turbines. To power the gas turbines, most LNG facilities utilize one or more internal (i.e., intermediate) process streams as fuel gas. Because the intermediate streams processed for fuel gas originate from several locations within the LNG facility, the final composition of the processed fuel gas can vary widely. As most process equipment requiring fuel gas (i.e., a gas turbine) is typically designed to operate with fuel gas having a reasonably constant composition, producing fuel gas having a widely varying composition can result in operational problems for the LNG facility.
One proposed solution for managing fuel gas streams having different compositions is to design the gas turbines to operate under multiple sets of conditions. For example, most gas turbines can be designed to have a dual fuel nozzle configuration to accommodate multiple possible fuel gas compositions without impacting turbine performance. However, gas turbines designed to operate with multiple fuel gas compositions are more expensive and more complex to operate than conventional gas turbines.
Thus, a need exists for a system for controlling fuel gas composition in an LNG facility in a way that minimizes capital and operating costs while maintaining or increasing plant operating flexibility.